This idea comes from the fact that the commands listed in the former post don't work for the initrd.lz which is in the "ISO/casper"directory, and fails with "prematured end of file", which means the archive is corrupted. (And it does work on an initrd.lz from another version of Ubuntu). So I have to change this archive anyhow.

I am not going to learn about coding, I was just seeking for information about the "/isolinux/bootlogo" file in the iso, because grep tells me there is a "jpg" file inserted inside the init file located in that bootlogo archive.

Not with the image, but copying the file from the Ubuntu Plymouth text theme into a new directory having for name bento-theme-text, changing the name of the file and some of the content, then use the command lines from that page:

The very first two screens in the live have their colors in the isolinux directory. One of the main background colors, once the images are changed, are setup in the gfxboot.cfg file. So if the background is still half purple, it has to be setup there.